Copyright not making sence

So im not understanding how this tiktok copyright stuff works. First of all when i post a video of a song or mv or whatever even if its HEAVILY altered, audio altered and i dont even post the full song just a part of it, no matter what i try its immediately copyright and muted but at the same time i see other people do it, like pleanty of videos where they dont alter it at all like full songs, or music videos and it doesnt get copyrighted tf? Makes 0 sence and no not people with rights to the audio just random fan accounts ect.. So it's inconsistent and doesnt make any sence. Can anyone explain this?